1. Field of the Invention
The present invention relates to a method for purifying a nitric-acid U/Pu solution of contaminants.
2. Description of the Prior Art
Such nitric acid solutions are produced, for instance, in wet scrap recycling in a fuel assembly factory. They must be recycled into the conversion process, i.e. into the process for manufacturing nuclear fuels and fuel pellets from UO.sub.2 and PuO.sub.2, respectively. The impurities contained therein, which are due, for instance, to the material of the reaction vessels as well as of the piping and are therefore predominantly of a metallic nature, must first be removed. The normal impurities consist of iron and chromium; in the case of plutonium-containing nuclear fuels, americium is added, a decay product of plutonium which is produced during the storage of plutonium-containing nuclear fuels but must not be incorporated, as a neutron poison, into nuclear fuel pellets which are to be freshly produced.
In extended storage of PuO.sub.2 -powder, it is likewise necessary to separate the americium that has built up. This can be done by the same method.
According to the present technique, for instance, the known Purex process, impurities are removed from uranium/plutonium solutions by extraction methods. Normally, a separation into a uranium solution and a plutonium solution takes place at the same time. With the extraction method, however, the use of organic, flammable solvents is necessary, which should be avoided as far as possible in the processing of plutonium in glove boxes; the fire hazard must be minimized.
Ion exchangers from processes for the purification of plutonium have also been used in many applications. These are mostly anion exchangers which are charged with strong nitric-acid solutions.
The plutonium (IV) is present in that case as a nitrato complex and remains in the ion exchanger column, while the impurities such as americium, uranium and heavy metals pass through the column. Thus, a separation of the uranium and plutonium comes about and the plutonium must be elutriated again with large amounts of diluted acid.
If cation exchangers are used, the plutonium (III) with all other metal cations is retained in the exchanger columns and uranium (VI) passes through the column as anion complex. Thus, a separation of uranium and plutonium again comes about, which latter is further loaded with all metallic impurities, as mentioned at the outset. Here, too, the plutonium must be elutriated with large amounts of diluted acid.
This state of the art thus requires in any case a rather large amount of equipment, especially since the impurities still must be separated from the uranium or plutonium, respectively.
Since uranium as well as plutonium are used in their oxidic form as nuclear fuel, especially also as mixed oxides, the problem arose to remove from their solutions only the impurities and to take them to a waste processing system and to recycle the so purified solution into the conversion process.